Axonal regeneration after injury, inflammatory attacks, or neurodegenerative diseases within the mammalian central nervous system (CNS) is almost always impossible; the outcome depends on the balance between the intrinsic ability of the nerve fibers in the CNS to re-grow, and the inhibitory factors within the CNS, localized in the microenvironment of the lesion or damage site, which actively prevent the re-growth, and thus the regeneration of the injured fiber tracts.
It has been established that CNS myelin, generated by oligodendrocytes, and the lesional scar are the most relevant non-permissive structures for axonal growth in the early phase of an injury, by causing growth cone collapse and neurite growth inhibition in vitro as well as in vivo, thereby resulting in direct inhibition of axon regrowth. RGM proteins, major inhibitory factors on CNS myelin and scar tissue have been identified (Monnier et al., Nature 419: 392-395, 2002; Schwab et al., Arch. Neurol. 62: 1561-8, 2005a; Schwab et al. Eur. J. Neurosci. 21: 1569-76, 2005 b; Hata et al. J. Cell Biol. 173: 47-58, 2006; for reviews see: Mueller et al., Philos. Trans. R. Soc. Lond. B Biol. Sci. 361: 1513-29, 2006; Yamashita et al. Curr. Opin. Neurobiol. 17: 29-34, 2007). RGM proteins are up-regulated at damage or lesion sites in humans dying from brain trauma or ischemic insult, (Schwab et al., Arch. Neurol. 62: 1561-8, 2005a) and are up-regulated at lesion sites in rats with spinal cord injury (Schwab et al. Eur. J. Neurosci. 21: 1569-76, 2005 b; Hata et al. J. Cell Biol. 173: 47-58, 2006 for review see: Mueller et al., Philos. Trans. R. Soc. Lond. B Biol. Sci. 361: 1513-29, 2006; Yamashita et al. Curr. Opin. Neurobiol. 17: 29-34, 2007). In addition, first data using clinical samples from Multiple sclerosis patients and healthy persons suggested that human RGM A is up-regulated in cerebrospinal fluid of patients suffering from MS (data not shown).
To evaluate the regeneration-promoting potential of an RGM A-specific polyclonal antibody, the antibodies were administered in a moderate-to-severe model of spinal cord injury, where approximately 60% of the spinal cord at thoracal level 9/10 was transected. The histological examination revealed that such a lesion severed all dorsal and lateral fibers of the corticospinal tract. The RGM A—specific polyclonal antibody given locally via pump for two weeks induced long-distance regeneration of injured nerve fibers (Hata et al., J. Cell Biol. 173: 47-58, 2006).
Hundreds of nerve fibers extended past the lesion site and the longest fibers regenerated for more than 10 mm beyond the lesion, whereas no regenerating fibers were found distal to the lesion in control antibody-treated animals. The functional recovery of the anti-RGM A treated rats was significantly improved in comparison with control-antibody treated, spinally injured rats, thereby proving that RGM A is a potent neuroregeneration inhibitor and a valuable target for stimulating recovery in indications characterized by axon damage or nerve fiber injury (Hata et al., J. Cell Biol. 173: 47-58, 2006; Kyoto et al. Brain Res. 1186: 74-86, 2007). In addition neutralising the RGM A protein with a function-blocking polyclonal antibody stimulated not only regrowth of damaged nerve fibers in the spinally injured rats but enhanced their synapse formation thereby enabling the reformation or restoration damaged neuronal circuits (Kyoto et al. Brain Res. 1186: 74-86, 2007).
The rgm gene family encompasses three different genes, two of them, rgm a and b, are expressed in the mammalian CNS originating RGM A and RGM B proteins, whereas the third member, rgm c, is expressed in the periphery (Mueller et al., Philos. Trans. R. Soc. Lond. B Biol. Sci. 361: 1513-29, 2006), where RGM C plays an important role in iron metabolism. In vitro, RGM A inhibits neurite outgrowth by binding to Neogenin, which has been identified as an RGM receptor (Rajagopalan et al. Nat Cell Biol.: 6(8), 756-62, 2004). Neogenin had first been described as a netrin-binding protein (Keino-Masu et al. Cell, 87(2): 175-85, 1996). This is an important finding because binding of Netrin-1 to Neogenin or to its closely related receptor DCC (deleted in colorectal cancer) has been reported to stimulate rather than to inhibit neurite growth (Braisted et al. J. Neurosci. 20: 5792-801, 2000). Blocking RGM A therefore releases the RGM-mediated growth inhibition by enabling Neogenin to bind its neurite growth-stimulating ligand Netrin. Based on these observations, neutralizing RGM A can be assumed to be superior to neutralizing Neogenin in models of human spinal cord injury. Besides binding of RGM A to Neogenin and inducing neurite growth inhibition, the binding of RGM A or B to the bone morphogenetic proteins BMP-2 and BMP-4 could represent another obstacle to successful neuroregeneration and functional recovery (Mueller et al., Philos. Trans. R. Soc. Lond. B Biol. Sci. 361: 1513-29, 2006).
The RNFL is the innermost layer of the retina and is principally composed of axons from ganglion cell neurons that compose the optic nerve. The axons within the RNFL are not myelinated until they pass through the lamina cribrosa of the eye. This structural characteristic of the RNFL makes it an ideal tissue to examine neurodegenerative processes within the CNS because RNFL thickness reflects the contribution of axons without potential structural effects of myelin degeneration (Frohman et al., Arch. Neurol. 65(1): 26-35, 2008) (See, also FIG. 19).
Frisén, L. and Hoyl, W. F. reported for the first time of thinning of RNFL in patients with Multiple Sclerosis (MS) (Frisén, L. and Hoyl, W. F., Arch. Opthalmol. 92: 91-97, 1974).
In healthy individuals, the RNFL is only about 110-120 μm thick by the age of 15 years and most normal individuals will lose about 0.017% per year in retinal thickness which equates to about 10-20 μm over 60 years (Kanamori. A. et al., Ophthalmologica 217: 273-278, 2003). Contrary to this about 75% of patients with MS who had experienced acute optic neuritis (AON) showed 10 to 40 μm RNFL thickness loss within a period of only about 3-6 months following the initial inflammatory event. It is also reported that thinning of the RNFL below a level of about 75 μm causes a decay of visual function (Costello, et al., Ann. Neurol. 59: 963-969, 2006). The potential utility of the RNFL for the purpose of modeling neuroprotection in response to MS therapies was suggested by Frohman et al., who also describe optical coherence tomography (OTC) as a reproducible imaging technique allowing measurements of RNFL thickness (Frohman et al., see above; and Frohman et al.; Nature Clinical Practice Neurology 4: 12, 664-675, 2008).
Degeneration of the RNFL is also observed during the course of numerous other diseases like; diabetic retinopathy, ischemic optic neuropathy, X-chromosome linked retinoschisis, drug-induced optic neuropathy, retinal dystrophy, age-related macula degeneration, eye diseases characterized by optic nerve head drusen, eye disease characterized by genetic determinants of photoreceptor degeneration, autosomal recessive cone-rod dystrophy, mitochondrial disorders with optic neuropathy, i.p. Friedreich's ataxia, Alzheimer's disease, mild cognitive impairment (MCI) Parkinsons's disease, and Prion disease, i.p. Creutzfeld-Jakob, Scrapie, BSE (see also Sakata L M, et al. Clin. Experiment Ophtalmol. 2009, 37: 90-99; Morris R W et al. Optometry 2009, 80, 83-100; Kallenbach K. & Frederiksen J. Eur. J. Neurol. 2007, 14: 841-849; Trick et al. J. Neuroophtthalmol. 2006, 26: 284-295; Tantri A. et al. Surv. Ophtalmol. 49: 214-230).
There is a need in the art for a therapeutic approach allowing the direct treatment of RNFL degeneration. The problem to be solved by the present invention was, therefore, to provide means allowing the direct treatment, in particular neuroprotective treatment of RNFL degeneration as observed in a multiplicity of disease conditions.